1. Field of the Invention
The present invention generally relates to the art of machining, and, more particularly, to a coolant/lubricant for forming small, deep holes with high precision and surface finish. The new coolant/lubricant is especially formulated for use with high toughness and high strength metal alloys.
2. Description of the Related Art
Machine operations involving cutting processes such as milling, drilling, broaching, and the like require a coolant/lubricant to aid in the machining. Of particular interest are improved lubricants/coolants for applications involving (1) machining of high strength, high toughness metal alloys and (2) high load and high stress machining operations. The following description is directed to the broaching process; however, it will be understood that this is merely an exemplary process in the use of lubricants/coolants.
Machining of small, deep holes with high precision and surface finish is a problem which has persisted in the art. A small, deep precise hole can be defined as having a diameter of less than 12 millimeters, an aspect (depth/diameter) ratio of at least 5, a precision of ISO standard H6-H7, an angular tolerance of H6, a surface roughness of 0.2 to 0.4 micrometer, and a bore out-of-roundness, cylindrical out-of-roundness and taper which are within 1/3 to 1/2 of the tolerance.
Prior art methods for machining small, deep holes include drilling and expanding followed by rough and fine reaming, rough and fine boring, or boring and grinding. Other methods include honing and electron discharge machining (EDM). These prior art methods suffer from the drawbacks of multiple complex machining processes, extreme difficulty in obtaining satisfactory precision, surface finish and exchangeability, low productivity, poor quality control, high reject rate, and often conical deformation at the exit ends of the holes.
Broaching is a process for machining holes, slots, and grooves with high productivity compared to the methods described above. Broaching can be used for forming holes in numerous metals including low-carbon steel, low-carbon alloy steel, phosphor bronze, pure aluminum, stainless steel, titanium alloys, and other materials.
A broaching tool generally includes an elongated body on which a number of parallel cutting teeth are formed or attached. The diameters of the teeth progressively increase from the front end to the rear end of the tool by an increment known as the "rise", such that each tooth cuts slightly deeper than the previous tooth.
A basic broaching tool and method are described in U.S. Pat. No. 1,945,535, entitled "BROACHING TOOL", issued Feb. 6, 1934 to B. Schlitz. A method of fabricating a basic broaching tool is described in U.S. Pat. No. 4,498,361, entitled "BROACH MANUFACTURING METHOD", issued Feb. 12, 1985 to W. Grace.
Broaching as practiced conventionally has not achieved its potential for forming small, deep holes with high precision and surface finish. This is due to a number of fundamental problems which have remained unsolved.
As the broaching tool is forced through the workpiece, high friction and specific pressure between the front face of each cutting tooth and the compressed material ahead of the tooth generate a large amount of heat which results in the formation of a layer of material which clings to the front face of the tooth and is known as a "built-up edge".
Certain "sticky" materials such as stainless steel are particularly prone to the formation of built-up edges due to their high elasticity, percentage elongation, and plastic deformation characteristics. The frictional forces and pressures between the chips generated during broaching, the broaching tool, and the workpiece are especially high for these materials, causing chips to break away from the workpiece that cause scaling of the surface of the hole and further enabling the built-up edge to grow to an undesirably large size. This causes the diameter of the hole to progressively increase, and creates a "nibbled" surface finish with a high degree of roughness.
If the built-up edge grows to a large size and then fractures off, the hole will have a surface with band-shaped scaling. Because cooling and lubrication are relatively ineffective in the lower portion of a deep hole which is being formed by vertical broaching, the scaling bands generally appear in the lower half of the hole.
There are four aspects of a coolant/lubricant to consider:
(1) It lubricates the cutting edge/chip/workpiece interfaces so that the chips will slide over the cutting tool surfaces with a minimum of friction and therefor generate a minimum of frictional heat and cutting tool abrasion. The coolant/lubricant also prevents built-up at cutting edges and extends useful life of the cutting tool.
(2) It conducts away heat generated by the separation of the chips from the workpiece and also the heat generated by the cutting edge's trailing edge which slides over the workpiece surface.
(3) It must penetrate and adhere at all the interfaces between the cutting tool and parts. To achieve this, sufficient volume of the coolant/lubricant fluid and pressure is required.
(4) It must not be corrosive to surfaces.
Satisfying these requirements, a well-formulated coolant/lubricant adds greatly to the production of smooth cutting surfaces and long cutting tool life.
Ineffective cooling and lubrication not only result in poorer quality holes due to occurrence of build-up edges, but also fail to protect the broaching tool itself from wear. The large heat concentration at the cutting point of the tool causes loose chips to fuse to the cutting tool edge, eventually blunting the tool. Once begun, the deterioration of a blunted broaching tool accelerates with use because a blunted tool generates more heat and results in more loose chips for fusion to the tool edge. Consequently, the service life of the broaching tool is shortened by inadequate lubrication and cooling, and the holes machined by the worn, blunted broaching tools have rough surface finishes and are generally of poor quality.
Prior art lubricants and coolants, including conventional cutting oils such as engine oil, spirdel oil, sulfurizing oil, and emulsions, are incapable of adequately preventing built-up cutting edges and reducing the frictional forces, temperatures, and pressures created during broaching small, deep holes. This lack of effective lubrication and cooling for broaching operations has limited the precision and surface finish of holes formed by broaching and has shortened the service life of broaching tools.
A coolant/lubricant specifically designed for the rigors of the broaching process is described in a pending application entitled "HIGH PRECISION, HIGH SURFACE FINISH BROACHING METHOD, TOOL, AND COOLANT/LUBRICANT", application Ser. No. 08/083,244, filed Jun. 24, 1993, in the name of Lin-Sen Yuan, one of the present inventors. The coolant/lubricant of that application comprises a molybdenum disulfide power dispersed in a liquid suspension of soap and water. In an alternative embodiment claimed within that application, the liquid suspension also includes kerosene, chloroparaffin, and carbon tetrachloride (CCl.sub.4). The use of CCl.sub.4 is to minimize sticking and prevent built-up cutting edges.
While the coolant/lubricant of that application overcomes the limitations of prior art lubricants and coolants described above regarding the broaching process, its use of the toxic chemical CCl.sub.4 is a concern for environmental and safety reasons. Moreover, the use of CCl.sub.4 is heavily regulated due to its toxicity, so that the cost of using CCl.sub.4 is effectively increased.
Further, the two classes of lubricants/coolants mentioned above function well only for different types of metals. The moly/soap/water formulation functions well for non-alloy metals, such as carbon steel, copper, etc., while the coolant/lubricant using CCl.sub.4 functions well for alloy metals, such as stainless steel.
Thus, a need remains for a non-toxic coolant/lubricant that can provide substantially the same level of lubrication and cooling in the harsh environment of machinging operations involving cutting currently achieved by molybdenum disulfide power dispersed in a liquid suspension of soap, CCl.sub.4, and water. The coolant/lubricant should also be convenient to store and transport.